[meteorite-list] Sky & Telescope News Bulletin - March 16, 2001

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:39 2004
Message-ID: <200103162156.NAA06322_at_zagami.jpl.nasa.gov>

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Were a small asteroid to hit the Moon, could we see the impact with
the naked eye? In his chronicles of medieval life, Gervase of
Canterbury described a dramatic event witnessed on the evening of June
18, 1178:

"Now there was a bright new Moon . . . and suddenly the upper horn
split in two. From the midpoint of this division a flaming torch
sprang up, spewing out . . . fire, hot coals, and sparks . . . The
body of the Moon which was below writhed . . . throbbed like a wounded
snake. Afterwards it resumed its proper state. The phenomenon was
repeated a dozen times or more. [Finally] the Moon . . . along its
whole length took on a blackish appearance."

In 1976 geologist Jack B. Hartung (State University of New York)
proposed that this passage describes the creation of Giordano Bruno, a
relatively young, 22-kilometer-wide crater near the Moon's northeast
limb. Hartung reasoned that, seen from Earth, this brightly rayed
crater appears near the midpoint of the young crescent Moon.
Astronomers were quick to counter that on the date in question the
Moon was only 1.3 days past new and thus too near the Sun to be easily
visible at all. Also, Gervase's witnesses claimed to have seen the
"flaming torch" many times, which sounds a lot more like the ordinary
atmospheric distortions often seen near the horizon. Still, Hartung's
hypothesis has made its way into many astronomy books and articles. It
proved difficult to confirm or refute because data on Giordano Bruno
and its surroundings were limited.

Now a new analysis demonstrates that a cratering event could not have
happened in 1178. Paul Withers (University of Arizona) finds that an
impact large enough to create a 22-km crater would likely have
showered Earth with 10 million tons of ejected fragments -- perhaps a
trillion bright meteors in all -- during the days that followed. "A
meteor storm as impressive as this and lasting for a week would have
been considered apocalyptic by all medieval observers," Withers
comments. Yet no mention of such displays appears in English,
European, Arabic, or Asian chronicles of the era.

Laser-ranging experiments during the 1970s revealed that the Moon nods
back and forth by a tiny amount ("free libration"), suggesting to
Hartung's supporters that the globe was still reverberating from the
impact. But Withers notes that a reanalysis of the laser-ranging data
later showed that the slight oscillation arises instead from fluid
motions deep in the lunar interior. Furthermore, while Giordano Bruno
is indeed the youngest crater of its size anywhere on the Moon,
multispectral images from the Clementine spacecraft show that this
impact site has to be much older than 800 years. Details of Withers's
analysis will appear in the April issue of Meteoritics.


At last year's Lunar and Planetary Science Conference, held each March
in Houston, Texas, meteorite specialists were salivating over the
Tagish Lake meteorite, which had dropped as a hail of fragments onto
the Yukon's winter wilderness just two months before. Within days of
the fall, local outdoorsman Jim Brook carefully collected nearly a
kilogram of icy fragments and stashed them in his freezer. Later a
team of Canadian geologists and volunteers scoured the lake's frozen
surface to collect as much of the fragile interplanetary material as
possible before the spring thaw swallowed up the remaining pieces.
Remarkable as much for the rapid, textbook recovery effort as for the
stones' black, carbon-rich texture, Tagish Lake was hailed as the most
important find in some 30 years.

A year later, the Tagish Lake fall is still causing a scientific buzz
because its unique composition, forged at the very beginning of the
solar system, defies easy explanation. For example, some of its dark,
crumbly interior is riddled with carbonate minerals created when
liquid water percolated through the rock multiple times. Yet adjacent
sections bear no carbonates or other traces of water's influence at
all. And though chemists would have bet money that the black stones
would have teemed with exotic hydrocarbon compounds, analyses turned
up a disappointing yield -- a thousandth the organic content of
Murchison, a similarly carbon-rich meteorite that fell in 1969. "We
were hoping to find all these amino acids," laments Iain Gilmour (Open
University), "and they're just not there."

What Gilmour and others have identified are puzzling clues to the
meteorite's origin. Some of the organic components mimic the nitriles
and other aromatic species known to exist in molecular clouds. So
might Tagish Lake have an interstellar origin? Or, as Takahiro Hiroi
(Brown University) speculates, are these pieces of one of the dark,
carbon-rich "D-type" asteroids that lurk in Jupiter's vicinity? More
than one specialist openly questioned whether this find could
represent chunks of a comet's nucleus. "There are no real conclusions
yet," says Sandra Pizzarello (Arizona State University). "This
meteorite is extremely difficult to study."


Copyright 2001 Sky Publishing Corporation. S&T's Weekly News Bulletin
and Sky at a Glance stargazing calendar are provided as a service to
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Received on Fri 16 Mar 2001 04:56:29 PM PST

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